The invention relates to an energy storage device, and in particular to an electrochemical energy storage device that can be charged multiple times. The invention further relates to a method for operating the aforementioned energy storage device.
So as to be able to store larger amounts of electric energy cost-effectively, for example for the stationary intermediate storage of power, reversibly-operating electrochemical storage devices having suitable energy densities, which can be scaled to large sizes and are preferably easy to produce are required. An electric battery shall be understood to mean an interconnection of multiple identical galvanic cells or elements. Strictly speaking, the term “battery” refers only to non-rechargeable so-called “primary cells” or “primary elements.” In contrast, “accumulators” is the term for rechargeable “secondary cells” or else “secondary elements.” However, use of the term “battery” has become somewhat lax of late, so that within the scope of the present invention the term “battery” is also used hereafter for rechargeable energy storage devices in a more generalized manner.
In principle, different battery types exist for different requirements, which differ in various respects ranging from the voltage curve to durability, and should be used accordingly.
The different battery types generally employ different storage materials (active materials), which may be present in at least one reduced form and one oxidized form (redox pair). With the aid of this active material, excess electric energy (such as from wind or solar energy) can be used to reduce the oxidized form of the active material, for example a metal oxide. The storage device is charged in the process. The reduced active material, such as a metal, can then be re-oxidized again as needed, thereby releasing electric energy. The storage device is discharged.
The active material used (redox pair) determines the rated voltage of the cell, while the amount of active material influences the energy content of the cell.
In addition to the generally known and customary types, such as zinc-carbon batteries, alkali-manganese batteries, or rechargeable lead, NiCd, NiMH or Li-ion batteries, a variety of designs of batteries have also become known as energy storages devices, which will be briefly outlined hereafter.
One design involves the redox flow batteries, as shown schematically in FIG. 1. In these, the electric energy is stored in chemical compounds (active material), which at room temperature are each present in dissolved form in a solvent. For this purpose, the electrolytes comprising the active material each respectively circulate in one of two circuits separated by a membrane. The ion exchange takes place via the membrane. The cell voltage in these systems is normally between 1.0 and 2.2 V. The solvents used are either inorganic or organic acids. Compounds made of titanium, iron, chromium, vanadium, cerium, zinc, bromine and sulfur are known active materials (redox pair).
Since the electrolytes comprising the energy-storing compounds can be stored outside the cell in separate tanks, this battery type is one example of electrochemical energy storage devices which have the advantage of allowing for varying and scaling the amount of energy by way of the electrolyte volume, and varying and scaling the power by way of the size of the electrode surface, independently of one another. A further advantage is that practically no self-discharge takes place when the system is idle.
Compared to other storage technologies, the redox flow battery has a high efficiency, allows self-discharge to be reliably avoided, and has a long life span since, among other things, the electrode material, which is usually graphite, is not directly involved in the electrochemical reaction of the electrolyte, and thus does not degenerate. However, the energy densities achieved thus far with redox flow batteries are still comparatively low.
Furthermore, various additional apparatus-related devices, such as pumps and the like, are needed for managing operations, which must be suitable for use of the electrolytes that are generally corrosive. Furthermore, large volume flows must be moved or controlled given the typically low solubility of the active materials in the solvent, which is usually water.
Another battery type is the high-temperature liquid metal battery. A well-known example of this is the sodium-sulfur system, for example. For the reactants, this form of battery usually employs two liquid materials serving as the electrodes, separated by a solid electrolyte. The operating temperature typically ranges around 700° C.
The high temperatures are necessary to bring the electrodes into a liquid state and render the ceramic electrolyte conductive. The electrodes can only be involved in the charging and discharging reactions in the liquid state. Such batteries can be used, for example, as stationary energy storage devices in wind and solar power plants.
Breakage of the electrolyte constitutes a critical incident in terms of safety in this type of battery, and may subsequently result in an uncontrolled reaction between the reactants, and thus in an uncontrolled release of energy.
In addition to these, a further liquid metal accumulator from the USA is known, in which the anode is made of magnesium, the electrolyte is made of the molten salt electrolyte MgCl2—KCl—NaCl, and the cathode is made of antimony (Sb). The operating principle is apparent from FIG. 2.
The composition and mode of operation of the aforementioned liquid metal accumulator are as follows: During heating to approximately 700° C., the materials, which are a mixture of ground magnesium and antimony metals, melt together with the MgCl2—KCl—NaCl salt mixture and, by virtue of the differing specific densities, form three horizontal layers.
The uppermost layer comprises the pure magnesium and serves as the negative electrode. The bottom layer is composed of a magnesium-antimony alloy, which forms the positive electrode of the accumulator. A salt layer composed of magnesium, potassium, sodium and chlorine forms the intermediate layer, constituting the electrolyte.
During the charging process, electrons find their way into the top magnesium electrode layer. At the same time, positively charged Mg ions form, releasing electrons from the magnesium-antimony alloy of the bottom electrode layer, and migrate through the electrolyte, likewise to the top magnesium layer, where they form metallic magnesium, accepting an electron. Conversely, during discharge, electrons are tapped in the top electrode, and Mg ions, which are again positively charged, migrate from the top magnesium electrode layer through the electrolyte back into the bottom layer.
Such an accumulator cell is suited for current densities of up to 200 mA/cm2. In general, the voltages of this battery type are considerably less than 1 V.
Another interesting alternative is metal-air batteries. Metal-air batteries are essentially batteries in which either (i) air is transported by an appropriate ion conductor and reacted there with a solid material (active material), or (ii) the solid material (active material) is first ionized and transferred into the electrolyte, then transported from there to the counter electrode, and oxidized there in an oxygen-containing medium.
Examples of the embodiment cited in (i) are described in the patent applications US 2012/0328972, US 2011/0033769 A1, WO 96/23322 and WO 2013/093 044 A1, for example.
In some lithium batteries, metallic lithium serves as the anode. In essence, this participates completely in the electrochemical reaction. One problem is posed by the growth of lithium metal during charging: it does not grow as a planar layer, but in the manner of a directed network, known as dendrite growth. These needle-like structures can cause undesirable short circuits in a battery cell.
One well-known example of lithium-metal-containing batteries is the rechargeable lithium-air battery, for example. More recent research is also directed to replacing lithium with sodium or zinc.
One problem with the metal-air batteries according to (i) is that of suitably bringing oxygen ions to the metal serving as the active material, while preventing damage to the battery from increases in the volume of the metal during oxidation, and preventing phases from forming that are not electrically conducting and considerably slow the further desired reaction.
FIGS. 3a to 3d (using the same schematic style as FIG. 5) show that there are four different variations for Li-air batteries alone, in terms of the design of the electrolyte: non-aqueous electrolytes, aqueous electrolytes, hybrid electrolytes, and solid electrolytes. All designs have in common that solid lithium metal is used as the anode material, and oxygen is used as the oxidizing agent at the cathode, as is apparent from FIGS. 3a to 3d. LiSICON denotes a Li super-ionic conductor.
The problem that can arise with the metal-air batteries according to (ii) is that all of the active material, which generally is metal, must first be ionized, and subsequently all of this material must be moved through the electrolyte. In place of the active material, an empty space is thus created in the remaining metal lattice, which must be appropriately filled again during charging. Moreover, problems with the electrical contact between the material and the current tap and between the material and the electrolyte may arise at this location. While metal that is again deposited during charging may grow in the intended location, it cannot be excluded that this may grow in another form, such as in the form of dendrites, instead of in the form of a compact layer. Moreover, the phase that forms at the counter electrode must not be overly thermodynamically stable, since such a phase frequently no longer has reversible properties, which is a disadvantage in the reverse reaction. In the case of a lithium-air system, for example, lithium peroxide would be a suitable phase, while in contrast Li2O is one example of a very stable phase.
Rechargeable lithium-ion batteries have had widespread success in recent years. They can already be found in many mobile devices. In addition to hybrid and electric vehicles, their field of application also includes the potential storage of power from wind or solar energy plants. Still, these batteries are not yet able to satisfy several requirements, specifically when it comes to energy storage density, and therefore many efforts are underway to explore alternative storage materials. The organic electrolytes used at present are not chemically or thermally stable and frequently also react fiercely with water or oxygen.
In the context of batteries or accumulators, capacity is generally understood to mean the maximum charge these are able to store. This is frequently stated as the product of electric current and time (such as in Ah). Energy within the scope of the invention shall be understood to mean the product of voltage (unit: volt) and charge (unit: ampere hours, for example), wherein this additionally may also be expressed in the form of energy density relative to the mass or the volume of the battery. For this reason, watt hours per kilogram (when based on mass) or watt hours per cubic meters (when based on the volume) is the unit used for energy density.